In the past Interior "C" Enamels which yield opaque films, U.S. Pat. No. 3,450,656, Col-1, Line 33-38, have been used to coat the interior of vegetable cans which contain wet foodstuffs. These "C" enamels, single package or two package, bake to give an opaque or milky film.
The zinc oxide pigment plays a two-fold role in protecting the appearance of the container and the packed food. Sulfur compounds in food may react with the ferric ions in the container's surface producing black iron sulfide. In the presence of zinc oxide, the sulfur compounds react preferentially with the zinc oxide forming white, zinc sulfide. Also, the opaque nature of the film produced by dispersing zinc oxide pigment in the varnish or enamel provides a visual carrier or mask to any unsightly discoloration of the metal surface, if it should occur.
The milky appearance is not always desireable to canners but is necessary in order to achieve good sulfide stain resistance. Many canners like the traditional clear gold appearance seen in many food cans, but they must accept the opaque look of a conventional "C" enamel for performance because the alternative was not available.
Prior to this invention the zinc oxide was incorporated simply by stirring or milling at room temperature a zinc oxide paste predisperson into the enamel or varnish of the coating.
This is a relatively inefficient manner for incorporating zinc oxide and required zinc oxide levels of 1% or more, U.S. Pat. No. 3,450,656, for acceptable sulfide stain resistance.
The metals used to fabricate cans for food packaging have changed over the years. Previously, cans had tin coating weights of 0.25 pounds of tin to 1.00 pounds of tin per 435.4 square feet of metal. Today, tin coating weights have been reduced to about 0.20 pounds of tin per 435.4 square feet of metal. This low tin weight steel is called L.T.S., or lightly tinned steel. This point is critical to both the packer and the coating supplier since it is easier to achieve good sulfide stain resistance on high tin weight steel than low tin weight steel. The thicker tin coating over the iron acts as a secondary or backup coating to protect the iron from the formation of iron sulfide. The current invention provides even greater stain resistance than the conventional "C" enamels since we are coating L.T.S. with a transparent, gold film.
In recent years, water-based coatings have found increasing use as interior coatings for food cans. The reason is changing laws and restrictions on solvent emissions which precludes the use of conventional solvent based coatings.
Some of the problems with most water based sanitary enamels are poor sulfide stain resistance and marginal adhesion on low tin weight cans, after steam processing. Steam processing is the process where the food which is being canned is heated 250.degree. F. in the can in order to sterilize it.
In an effort to cut costs, canners are not only using low tin weight metal, but are not subjecting the metal to the washing treatments necessary to remove surface contamination which causes adhesion failures.